Metal detector search coil

ABSTRACT

A device for detecting metal articles buried beneath the surface for example, includes a search coil at one end of a handle, with the handle carrying the detector circuit components. The search coil is an induction coil enclosed within an annular chamber formed from a plastic housing having the form of a spoked wheel for example, and which is formed from two mating housing halves joined in the plane of the coil chamber. The coil is made up of a number of turns of insulated wire wherein the insulation material has a low coefficient of thermal expansion, with the coil being sheathed in metal foil to provide a Faraday shield. The coil is supported within the housing annular chamber on pads of foam rubber-like material which yieldingly supports and restrains the coil within the housing annular chamber.

O United States Patent [191 [111 3,753,185

Mahan l Aug. 14, 1973 METAL DETECTOR'SEARCH COIL Primary Examiner-Thomas .l. Kozma [76] lnventor: William A. Mahan, P.O. Box 451, Atmmey Peter Murphy et Dallas, Tex. 75040 [22] Filed: Dec. 5, 1972 ABSTRACT [211 App]. 2 2 A device for detecting metal articles buried beneath the surface for example, includes a search coil at one end of a handle, with the handle carrying the detector [52] U.S. Cl 336/84, 324/3, 324/41, circuit components The Search coil is an induction coil 336/92 336/229 enclosed within an annular chamber formed from a [51] Int. Cl H01! 15/04, HOlf 27/02 plastic housing having the form of a spoked wheel for [58] Field of Search 336/84, 90, 92, 229, example and which is formed from two mating housing 336/232, 66; 324/41, 3 halves joined in the plane of the coil chamber. The coil is made up of a number of turns of insulated wire Rderenc Cited wherein the insulation material has a low coefficient of UNlTED STATES PATENTS thermal expansion, with the coil being sheathed in 3,662,255 5/1972 Garrett 324/3 m tal f il t provide a Faraday shield. The coil is sup- 3,'634,798 1/1972 Astleford, Jr. 336/92 ported within the housing annular chamber on pads of ,2 8/1966 karian 336/ foam rubber-like material which yieldingly supports 3,237,137 2/1966 Flaminio 336/229 X and restrains the coil within the housing annular chamber.

11 Claims, 7 Drawing Figures Patented Aug. 14, 1973 E" snail-life METAL DETECTOR sEARcucori. 1

. CKGRQU D AND. SUMMARY O TH E If INVENTION it This invention relates to dcvices.for. locating metalobjects buried in the ground forexarn'plefandv more particularl'y toa search coilstructure forsuch device to i ner thatthe oscil lator frequency will vary with a change in ind'uctance of the 'coilresulting from; proximity of 4 metal objects, with the change" of frequency being de- 1 tected to produce a signal indicating t he presence of such metal objects, It has been found thattlte inductor coil structure; and this frequency'drift neces I frequency ofthe oscillator circuit in which thesearch'coil is connected will change or drift in response to certain physical changes in the r sarilyproduces signals in th'ejdetector which are false *signals'in the sense that they are notresponsive to the presence of metal articl'es Such false signals', of course, would greatly reduce the reliability. of the detector de- ,vace. g v l f- 'i g i lt has been observed that such false signals are pro- 7 I duced by changes in the physical relation'of the several turns of wire'which make up the induc'ttir coil, for ex- 'amplc,'and also by compressive forces'acting onone or more of the turns of wire making up the inductor coil. It has further been'observed that pressure forces acting as well asadditionaliobjects thereof will beunderstood more-fully from the;following descriptionwhen riead in on'the turns of wirein'thelcoilresult from the'manner wire forming the inductor coil are tightly encased structure itself and the wire insulation mayexpand due to thermalexpansion'resulting in increase-of pressure onthe coil turns resulting in an'error signal. Sirnilarly, "as a result of thermal expansion, the search coil housing inwhich the inductor'coil mounted withina'n'enclosing housing. In some detector search vcoils{the-turnsof Z It is a further object of thisinvention to provide a search coilgstructu re for use with a metal detector to produce areliable device which is not subject to false signals due to changes in the ambient temperature.

These objects are accomplishedin a search coil v structure comprising a flathousingwhich defines an .annular'chamber having bottom a'nd top walls. An in: i duction coil. formed from a plurality of--.turns of insu "latedv'vire isdisposed within'the annular chamber, with t the turns being further'enclosed within asheath to forrn f 1, I

. an annular coil unit. Cushioning support pads are dis I osed in confronting relation withinythe annular housing chamber contiguous to the-bottom and top wa'lls, A

with,the confrontin'g'faces being spaced a distance-less thanthe diameter of the sheath coil unit. The coil unit is supported between thesesupport'pads in spaced rela- 1 tion to the. housing chamber walls.

Moreparticularlythe support p'adsmay day shield. The turns of wliregare preferably insulated pansion... I t The novel features and advantagesofthe invention,

connection with the accompanying drawing DRAWING "FIG. l is a'perspective view of a metal detector de- I vice havinga search coil-structure according .tothe ;present invention;

H0. 215 a plan view of the search can housing; H6. 3 is a side elevation view of the search coil housing of FlG. 2-,

HO. 4 is a fragmentary view of the search coilhousing and associated inductor coil structure as viewed in may seek to expand or radially enlarge to a greaterex- ,tent than the coil wires and associated insulation resulting in forces actingon the coil. wires tolproduce a false signal. V i J n 1 With' other'types of coil mounting withinthe coil housing, the relative movement caused by thermal'ex pansion changes the relative'position of the coil'wires to produce false signals. Anothercause of relative wire movement or distortion of coil arrangement may be the tion, due totemperature increase. I

mize external physical forces acting on'the wire turns tant false signals.

I It is another object of this invention to provide a which would tend to produce frequency drift and resul-j search coil, structure wherein the entire assembly can be manufactured readily and economically.

softening of the 'wire insulation, such as vinyl insula I wheel including a rim portion 21, a central'hub portion is provided with a handle .theplane 4-4 of FIG. 3; Y

FIG. 5 is a fragmentary view of a portion of the iii-the plane 6 6 of FIG. 2; and v i FIG. 7 is a-fragrnentary sectional view of the search V coil housing as viewed in the plane 77 of FIG.- 2.

DESCRIPTION'OF THE PREFERRED EMBODIMENT FIG; 1' of the drawing is a perspective view of a metal detector 10 having a search coil structure 20 according,

;to the present invention; A detector includes a support- 7 v ing shaft or handle 11 which is secured to the search coil 20 by means of a flanged socket 1 2 suitably 'secured to the se'arch coil housing, and which. hasa recess for? receiving the lower end of the handle ll which is in turn retained-in the socket by any suitable means. A v circuit housing 13 is attached to the handle adjacent to its upper end and contains the major portion of the circuitry and controls for operating the detector 10. The

handle 11 projecting upward fromthe circuit housing manipulating the detector.

Thesea'rch coil structure is V which is illustrated as having the form of a spoked be fabricated froma foam rubber type'rnaterial'and are annular and coextensive with the walls of, the housingtchamber; The sheath maybe fabricatedofmetal foil to define a Faragrip l4 for convenience in enclosed in a housing 22 and three interconnecting spokes 24. While this 7 formed in any desired manner, such as by vacuum forming or by injection molding, to produce a'housing which is hollow in assembled form.'The above mentioned flanged socket 12 which may also be fabricated of a plastic material, is secured to the search coil housing at the hub portion in any suitable manner such as by cementing or riveting or both. Materials which are 7 especially suitable for the fabrication of the search coil housing are acrylonitrile butadiene styrene. (ABS),

acrylic coated acrylonitrile butadienestyrene, and high impact polystyrene. g

The rim portion 21 of the search coil housing defines an annular chamber within which is disposed the induction coil unit 30 which is the search coil for the circuit of the detector 10. This annular chamber, as best seen in cross-section in FIG. 6, is shown as being generally rectangular"in'cross-section and having top and bottom walls defined by the top and bottom housing portion 24a and 24b respectively.

Upper and lower annular supp ort pads 25a and 25b are disposed in the annular chamber adjacent to the housing top and bottom walls respectively. As shown in FIG. 6 of the drawing, these pads are generally rectangular in cross-section and generally coextensive with the width of the annular chamber; and the pads are secured within the housing chamber in any suitable manner. These pads are fabricated of a resilient cushion material which preferably is a foam rubber type material such as ethane foam rubber.

The induction coil unit 30 is made up of a plurality of turns of a continuous insulated wire 31 formed to a diameter corresponding to the diameter of the rim portion 21 of the search coil housing, with the turns of wire being encased within a sheath 32 of metallic foil such as lead foil or copper foil. The turns are completely encased in this metallic sheath 32 which is arranged on the coil to define a Faraday shield. For the purpose of forming this Faraday shield, the sheath 32 is provided with an. insulation joint defined by an insulation sleeve 33 disposed on the coil unit 30 opposite from the point where the coil lead wires emerge, namely at the housing spoke 23a. This insulation joint is indicated in FIG. 2 and is illustrated in FIG. wherein the telescoping endsof the sheath 32 are radially separated by the insulation sleeve 33.

The sheath 32 retains the wire turns 31 in a generally cylindrical bunch, as best seenin cross-section in FIG.

6; and the coil unit defined by the sheathed wires is retained within the annular chamber of the housing rim 21 by the support pads 25a and 25b, the confronting faces of which are spaced a part a distance less than the diameter of the coil unit. Accordingly the coil unit is supported or retained within the housing chamber by the cushioning pads in a manner to provide very light curely in spaced relation to thewalls of the housing chamber. With this cushioning type support, there will be minimal changes of pressure or force acting on the coil unit due to unequal expansion and contraction of I the search coil housing and search coil unit resulting front changes in temperature. This resilient cushioning support will also accommodate any relative radial enlargement or contraction of the coil unit relative to the housing. With this type of cushion mounting of the coil unit, it has been found that false signals resulting from variations in ambient temperature are very substantially minimized. I I

Another factor which appears to produce false or error signals inresponse to temperature changes is a type of insulation which is used on the turns of wire 31.

Where the insulation is of a material which has a relatively high coefficient of heat expansion and therefore significantly enlarges or contracts with changes in tem perature, this apparently has the effect of'changing the physical forces acting on the turns of wire within the coil unit and also of shifting the positions of the several wires in the unit to produce a false signal. Additionally, if the insulation material has a low meltingpoint and therefore softens substantially and relatively low tem- 7 peratures, this may result in shifting of wires within the coil unit. It has been discovered that when wire is used having a type of insulation which has a low coefficient of thermal expansion and whose physical characteristics do not change appreciably with extremechanges in temperature,v the occurence of false signals is greatly reduced. The wire insulation then should be of a material which has a low coefficient of thermal expansion Referring now to the connection of the induction to the remainder of the detector circuit, one end 31a of the coil wire 31 emerges from the coil unit adjacent to the housing spoke 23a and extends radially through this hollow spoke to be joined to the central post 41 of a co-- axial connector 40, which is mounted in an aperture in physical pressure or force on the coil unit, defining a form of floating support; yet the coil unit is retained sethe top housing member 24a adjacent to its hub portion 22. The other end of the induction coil wire 31 is connected, by means of a soldered joint 35, both to a ground lead wire,36 and to the metallic sheath 32. The ground lead wire is in turn connectedto the housing of the coaxial connector 40. Both of the coil lead wires 310 and 36 are enclosed by a metallic sheath 37 which I is connected to the metallic sheath 32 and defines an extension of the Faraday shield through the lead'wire spoke 230. By means of a complementary coaxial connector 42 and a coaxial cable 43, the induction coil is connected to the remainder v of the electrical circuitry contained in the circuit housing 13.

What has been described is a novel and improved search coil structure having features which serve to minimize or eliminate false signals which are somen times produced by the induction coil, resulting from physical forces acting on the coil and produced by relative expansion and contraction of the search'coil structural parts because of extreme variations in the ambient temperatures surrounding the search coil under conditions of use. While these novel structural features are very etfective for the intended purpose, they also enable the economic production of search coils to produce an effective and useful metal detector.

While a preferred embodiment of the invention has been illustrated and described, it will be understood by those skilled in the art that changes and modifications may be resorted to without departing from the spirit and scope of the invention.

What is claimed is: 1. For use with a metal detector, a search coil structure comprising a flat housing defining an annular chamber having bottom and top walls; an induction coil, formed from a plurality of turns of insulated wire, disposed within said housing annu lar chamber; a sheath enclosing said induction coil turns to form an annular coil unit; cushioning support pads disposed within said housing chamber contiguous to the bottom and top walls respectively; said support pads confronting each other, with the confronting faces being spaced a distance less than the diameter of said sheathed coil unit; and said coil unit being supported between said pads in spaced relation to the housing chamber walls. 2. The search coil structure set forthin claim 1 said sheath being fabricated of metal foil to define a Faraday shield. 3. The search coil structure set forth in claim 1 said support pads being annular and coextensive with said chamber walls. 4. The search coil structure set forth in claim 1 said housing being formed of mating members adjoined in the plane of said annular chamber; said housing members being fabricated from a plastic material.

5. The search coil structure set forth in claim 2 said housing providing support means adjacent to the center thereof and a radial channel for enclosing the induction coil lead wires; a coaxial connector mounted on said housing radial channel adjacent to the center thereof for connection to said lead wires; and a metal foil sheath enclosing said lead wires in said radial channel.

6. The search coil structure set forth in claim 1 said housing having the form of a spoked wheel having a rim portion defining said annular chamber; and having a hub portion for securing a supporting handle.

7. The search coil structure set forth in claim 1 said support pads being fabricated of a foam rubber type material.

8. The search coil structure set forth in claim 1 said induction coil turns being formed from wire having an insulation fabricated from a material having a low coefficient of thermal expansion.

9. The search coil structure set forth in claim 1 said induction coil turns being formed from wire having insulation fabricated from a material which remains firm at ambient temperatures.

10. The search coil structure set forth in claim I said wire insulation being fabricated from tetrafluroethylene.

11. The search coil structure set forth in claim 1 said wire insulation being fabricated from asbestos.

III t 

1. For use with a metal detector, a search coil structure comprising a flat housing defining an annular chamber having bottom and top walls; an induction coil, formed from a plurality of turns of insulated wire, disposed within said housing annular chamber; a sheath enclosing said induction coil turns to form an annular coil unit; cushioning support pads disposed within said housing chamber contiguous to the bottom and top walls respectively; said support pads confronting each other, with the confronting faces being spaced a distance less than the diameter of said sheathed coil unit; and said coil unit being supported between said pads in spaced relation to the housing chamber walls.
 2. The search coil structure set forth in claim 1 said sheath being fabricated of metal foil to define a Faraday sHield.
 3. The search coil structure set forth in claim 1 said support pads being annular and coextensive with said chamber walls.
 4. The search coil structure set forth in claim 1 said housing being formed of mating members adjoined in the plane of said annular chamber; said housing members being fabricated from a plastic material.
 5. The search coil structure set forth in claim 2 said housing providing support means adjacent to the center thereof and a radial channel for enclosing the induction coil lead wires; a coaxial connector mounted on said housing radial channel adjacent to the center thereof for connection to said lead wires; and a metal foil sheath enclosing said lead wires in said radial channel.
 6. The search coil structure set forth in claim 1 said housing having the form of a spoked wheel having a rim portion defining said annular chamber; and having a hub portion for securing a supporting handle.
 7. The search coil structure set forth in claim 1 said support pads being fabricated of a foam rubber type material.
 8. The search coil structure set forth in claim 1 said induction coil turns being formed from wire having an insulation fabricated from a material having a low coefficient of thermal expansion.
 9. The search coil structure set forth in claim 1 said induction coil turns being formed from wire having insulation fabricated from a material which remains firm at ambient temperatures.
 10. The search coil structure set forth in claim 1 said wire insulation being fabricated from tetrafluroethylene.
 11. The search coil structure set forth in claim 1 said wire insulation being fabricated from asbestos. 